Add trains (#1159)

## Description:

Add a rail network to handle train stations/railroad between structures.

Changes:
- `RailNetwork` is responsible for the train station graph. Use it to
connect new `TrainStations`
- A `RailRoad` connects two `TrainStation`
- No loop possible in the rail network
- Train stations handles its railroads
- Added a layer to draw the railroads under the structures

#### Clusters
- To speed up computations, each `TrainStation` references its own
cluster
- A cluster is a list of `TrainStation` connected with each other,
created by the `RailNetwork` when connecting the station
- Train stations spawn trains randomly depending on its current cluster
size
- A `TrainStation` decides randomly of the train destination by picking
one from the cluster

#### Production building:
- Added a factory which has no gameplay impact currently. _To be
discussed._

#### Train stops:
- When a train reaches a factory, it's filled with a "cargo". The loaded
trains has no impact currently. _To be discussed._
- When a train reaches a city, the player earn 10k gold
- When a train reaches a port, it sends a new tradeship if possible
- If a destination/source is destroyed, the train & railroad are deleted
too


https://github.com/user-attachments/assets/42375c17-9e04-4a42-98d0-708c81ffd609


https://github.com/user-attachments/assets/fbecdb53-a516-4df8-87fb-1f9a62c4efa0



## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

---------

Co-authored-by: Scott Anderson <scottanderson@users.noreply.github.com>

src/client/graphics/layers/StructureLayer.ts

@@ -7,6 +7,7 @@ import { Layer } from "./Layer";
 import { UnitInfoModal } from "./UnitInfoModal";
 
 import cityIcon from "../../../../resources/images/buildings/cityAlt1.png";
+import factoryIcon from "../../../../resources/images/buildings/factoryAlt1.png";
 import shieldIcon from "../../../../resources/images/buildings/fortAlt2.png";
 import anchorIcon from "../../../../resources/images/buildings/port1.png";
 import MissileSiloReloadingIcon from "../../../../resources/images/buildings/silo1-reloading.png";
@@ -72,6 +73,12 @@ export class StructureLayer implements Layer {
       territoryRadius: BASE_TERRITORY_RADIUS * RADIUS_SCALE_FACTOR,
       borderType: UnitBorderType.Round,
     },
+    [UnitType.Factory]: {
+      icon: factoryIcon,
+      borderRadius: 8.525,
+      territoryRadius: 6.525,
+      borderType: UnitBorderType.Round,
+    },
     [UnitType.MissileSilo]: {
       icon: missileSiloIcon,
       borderRadius: BASE_BORDER_RADIUS * RADIUS_SCALE_FACTOR,